Condensing steams heat flow

Steam-Heated Exchangers Steam, the most common heating medium, transfers its latent heat in condensing, causing heat flow to be proportional to steam flow. Thus, a measurement olsteam flow is essentially a measure of heat transfer. Consider raising a hquid from temperature Ti to To by condensing steam ... 

Fig. 5. Relative energy flows showing power generation and heat losses in GJ/h for (a), boiler only (b), boiler + steam turbine (c) combined cycle employing gas turbine and (d), condensing steam for power only. To convert /h to Btu/h, multiply by 0.95 x 10 . ...

The value of the coefficient of heat transfer from steam to sheet is determined by the conditions prevailiug on the inside and on the surface of the dryers. Low coefficients may be caused by (1) poor removal of air or other noncoudeusables from the steam in the cyhn-ders, (2) poor removal of condensate, (3) accumulation of oil or rust on the interior of the drums, and (4) accumulation of a fiber lint on the outer surface of the drums. In a test reported by Lewis et al. [Pulp Pap. Mag. Can., 22 (Februaiy 1927)] on a sulfite-paper diyer, in which the actual sheet temperatures were measured, a value of 187 W/(m °C) [33 Btu/(h ft" °F)j was obtained for the coefficient of heat flow between the steam and the paper sheet. 

If steam condenses on a surface, there is no boundary layer the resistance to heat flow is due to scale, metal thickness, and the condensed liquid layer, resulting in a high heat transfer factor. A thin layer of air or other noncondensing gas forms at the surface through which the steam diffuses. The heat transfer factor diminishes rapidly but is considerably higher than in dry convection. 

When steam at the saturation temperature contacts a surface at a lower temperature, and heat flows to the cooler surface, some of the steam condenses to supply the energy. With a sufficient supply of steam moving into the volume that had been occupied by the steam now condensed, the pressure and temperature of the steam will remain constant. Of course, if the condensate flows to a zone where it is no longer in contact with the steam it can cool below steam temperature while supplying heat to a cooler surface. 

This heat flow implies the condensing of some 550000/ 2163.3 or 254.6kg/h of steam.) From Equation (22.1) ... 

Where extraction turbines are employed, it is most efficient to operate at the lowest level of condensing possible (the highest level of extraction). This occurs when steam from the turbine is extracted and directed by various takeoffs, either to air or water heaters or to industrial processes requiring steam. This practice occurs because no heat is lost from the steam flowing from the throttle to the extraction takeoff (whence it can be delivered for beneficial use by the industrial process), whereas over two-thirds of the heat flowing from the throttle to the surface condenser is ultimately rejected, so efficiency suffers. 

In the previous discussion it has been assumed that the vapour is a pure material, such as steam or organic vapour. If it contains a proportion of non-condensable gas and is cooled below its dew point, a layer of condensate is formed on the surface with a mixture of non-condensable gas and vapour above it. The heat flow from the vapour to the surface then takes place in two ways. Firstly, sensible heat is passed to the surface because of the temperature difference. Secondly, since the concentration of vapour in the main stream is greater than that in the gas film at the condensate surface, vapour molecules diffuse to the surface and condense there, giving up their latent heat. The actual rate of condensation is then determined by the combination of these two effects, and its calculation requires a knowledge of mass transfer by diffusion, as discussed in Chapter 10. 

A liquid hydrocarbon is fed at 295 K to a heat exchanger consisting of a 25 mm diameter tube heated on the outside by condensing steam at atmospheric pressure. The flow rate of the hydrocarbon is measured by means of a 19 mm orifice fitted to the 25 nnn feed pipe. The reading on a differential manometer containing the hydrocarbon-over-water is 450 mm and the coefficient of discharge of the meter is 0.6. 

The reactor residence time is about 45 minutes, a 95 per cent approach to equilibrium being achieved in this time. The ammonia is fed directly to the reactor, but the carbon dioxide is fed to the reactor upwardly through a stripper, down which flows the product stream from the reactor. The carbon dioxide decomposes some of the carbamate in the product stream, and takes ammonia and water to a high-pressure condenser. The stripper is steam heated and operates at 180°C, whilst the high-pressure condenser is at 170°C and the heat released in it by recombination of ammonia and carbon dioxide to carbamate is used to raise steam. Additional recycled carbamate solution is added to the stream in the high-pressure condenser, and the combined flow goes to the reactor. 

When we developed the model for the stirred tank heater, we ignored the dynamics of the heating coil. Provide a slightly more realistic model which takes into consideration the flow rate of condensing steam. 

In most jacketed reactors or steam-heated reboilers the volume occupied by the steam is quite small compared to the volumetric flow rate of the steam vapor. Therefore the dymamic response of the jacket is usually very fast, and simple algebraic mass and energy balances can often be used. Steam flow rate is set equal to condensate flow rate, which is calculated by iteratively solving the heat-transfer relationship (Q = UA AT) and the valve flow equation for the pressure in the jacket and the condensate flow rate. 

In a Rankine power plant, the steam temperature and pressure at the turbine inlet are 1000°F and 2000 psia. The temperature of the condensing steam in the condenser is maintained at 60° F. The power generated by the turbine is 30,000 hp. Assuming all processes to be ideal, determine (1) the pump power required (hp), (2) the mass flow rate, (3) the heat transfer added in the boiler (Btu/hr), (4) the heat transfer removed from the condenser (Btu/hr), and (5) the cycle thermal efficiency (%). 

Figure 9 Steam-stripping flow diagram. The influent is heated by the stripper effluent before entering the stripping column near the top the liquid stream flows downward through the packing, and steam flows upward, carrying volatile compounds the overhead is condensed and liquid returned to the column volatile compounds are either recycled or incinerated (from Ref. 11).

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